As one of significant factors influencing an image quality of a printing apparatus, there is a gradation characteristic. The gradation characteristic is explained below with reference to FIG. 1. FIG. 1 represents a schematic flow of data processing executed by the printing apparatus. In the case of FIG. 1, input data is given as digital data of an ROB format. A bit length of each of the colors is given as 8 bits and a bit length of the colors is given as 24 bits in total. In this case, the respective colors of RGB have information on 256 gradations from 0 to 255.
A color converting unit 1 converts the digital data in the RGB format into four colors corresponding to ink colors, i.e., respective digital data (each including 8 bits from 0 to 255) of Y (yellow), M (magenta), C (cyan), and K (black).
A half toning unit 3 converts the digital data after color correction into driving data for heads 5 corresponding to the respective colors.
The heads 5 eject ink droplets on the basis of the driving data and form print image on a print medium.
It is assumed that the density of an output result with respect to the digital data (0 to 255) after this color conversion is required to have a linear relation shown in FIG. 2. Ideally, a relation between the digital data and the density is not limited to the linear relation.
As a numerical value representing density, besides optical density, various numerical values such as a value of L* of a Lab color space, an absolute value of ⅓th power of X, an absolute value of ⅓th power of Y, and an absolute value of ⅓th power of Z of an XYZ color space, and a read value by a scanner can be used.
Incidentally, the optical density is logarithmic representation of a degree of light not transmitted and reflected with respect to a certain portion of a photograph film, photographic paper, or the like. A minimum value of the optical density is 0.00 (entirely transmitted and reflected). A larger numerical value of the optical density indicates that an image is darker.
However, actually, the digital data and the density have a gradation relation of gradation saturated in a high density region as shown in FIG. 3.
Therefore, as shown in FIG. 4, a gradation correcting unit 7 is provided at a post stage of the color converting unit 1 to execute a correction operation for canceling the gradation characteristic shown in FIG. 3. An example of a gradation correction curve is shown in FIG. 5. According to this gradation correction, the digital data after color conversion and the density of the output result are corrected to satisfy the gradation relation shown in FIG. 2.
However, when the gradation correction indicated by the gradation correcting unit 7 is executed, as shown in FIG. 6, an actual number of gradations substantially decreases.
As one of methods of controlling the decrease in the number of gradations, there is a method of increasing the number of processed bits. For example, there is a method of changing 8-bit digital data into 10-bit or 12-bit digital data and processing the digital data.
However, this method has problems in that a large number of memories are necessary and processing speed falls.
A cause of these problems is that, when multi-gradation data is binarized by the error diffusion method, the gradation characteristic shown in FIG. 3 inevitably tends to appear.
To cope with these problems, the inventor proposes a method of improving the appearance of this gradation characteristic by applying the multi-value error diffusion method. The multi-value error diffusion method means an error diffusion method of changing three-value original image data to multi-value image data of about eight values and diffusing a density error caused in that case to peripheral pixels. In the multi-value error diffusion method, it is a general practice to determine, as boundary values, values obtained by equally dividing the number of gradations of the original image data by the number of multiple values.
However, a visual change in printing an image at a lowest level from a state in which no image is printed (level 0) is larger than a visual change in printing an image at a level 2 from a level 1. Therefore, when the level 0 and the level 1, the level 1 and the level 2, the level 2 and the level 3, and the level 3 and the level 4 are set at equal intervals, respectively, as shown in FIG. 7, a tendency that image density substantially shifts from an ideal state with respect to an input signal appears.
Therefore, the inventor proposes a method of optimizing boundary values for multi-value error diffusion such that a print result conforms to an ideal gradation characteristic curve (JP-A-2005-252633).
When this method is applied, a relation of image density to an input signal can be brought closer to an ideal relation as shown in FIG. 8. A gradation characteristic among boundary values may slightly shift with respect to a characteristic curve shown in FIG. 8. Therefore, in an actual printing system, a gradation correcting unit may also be used for the purpose of finely adjusting the shift.
However, in general, gradation characteristics of the respective print heads have fluctuation. Examples of a cause of the fluctuation include fluctuation in a nozzle diameter for ejecting ink droplets, fluctuation in the height of a liquid chamber, and fluctuation in heater performance.
Due to this characteristic fluctuation, there is a problem in that, even if the gradation characteristics shown in FIGS. 2 and 8 can be obtained for a certain head, the same characteristics are not obtained in another head.
JP-A-3-252269 discloses a method of solving this problem. In short, JP-A-3-252269 discloses a method of storing plural kinds of gradation correction data in a gradation correcting unit 7 and, on the other hand, storing information for selecting these gradation correction data in respective print heads to thereby use gradation correction data suitable for mounted heads during gradation correction.
However, the method of substantially correcting a gradation value using the gradation correcting unit in this way has a significant problem in that a realizable number of gradations substantially decreases as described above.